Several methods have been developed to electrically stimulate nerve fibers in the cochlea of a deaf person in a pattern roughly corresponding to longitudinal mechanical waves produced in the environment (which waves are heard as sound by persons with normal hearing) to thus produce the sensation of sound in the deaf person. The details and results of one such method are set forth in a document entitled "Cochlear Implants: Progress and Perspectives" edited by William F. House and Karen I. Berliner, and which is Supplement 91-Volume 91, Mar.-Apr. 1982, No. 2, Part 3 of the Annals of Otology, Rhinology and Laryngology, published by The Annals Publishing Company, 4949 Forest Park Boulevard, St. Louis, Mo. 63108, copyright 1982, the content whereof is hereby incorporated herein by reference.
Generally such methods involve implanting an internal electromagnetic coil connected by leads to active and ground electrodes under the skin covering the person's skull and positioning the electrodes at predetermined locations in or adjacent the cochlea. An externally worn adjustable transducer is then used to produce electrical signals in response to longitudinal mechanical waves produced in the environment and received via a microphone on the device, and to couple those electrical signals to an external electromagnetic coil positioned on the outside of the skin opposite the coil under the skin. Electrical signals corresponding to those produced by the transducer are produced in the internal coil by magnetic coupling between the coils, and those signals stimulate the cochlea via the electrodes. While the signals do not produce the same sound sensations from the mechanical waves that the waves produce via the normal human ear, they do produce a range of sound sensations that, with training, can be used by the person to help identify the source of the waves and in many cases to help the person understand human speech.
Implanting the coil with its leads and electrodes under the skin covering the person's head generally comprises (1) exposing the outer surface of the person's temporal bone behind the ear, which is typically done by cutting an arcuate slit in the skin, muscle, and other tissue overlaying that bone, separating, and laying back the flap thus produced; (2) making an opening in the temporal bone through the mastoid and facial recess portions of the temporal bone between the outer surface of the temporal bone and the round window in the cochlea; (3) forming a recess for the coil via a surgical drilling device in the exposed squamous portion of the temporal bone; (4) locating and attaching the internal coil in the recess; (5) positioning the ground electrode in the eustachian tube or some other location such as the opening in the mastoid or under the temporalis muscle; (6) positioning the lead terminating at the active electrode through the surgically prepared opening with the electrode at a predetermined position relative to the cochlea (e.g., which position may be at the outside surface of the round window of the cochlea or inside the cochlea with the lead passing through the round window); (7) anchoring the leads in place through the use of an acceptable adhesive or by packing tissues from the head around them in the opening; and (8) then replacing the flap of skin, muscle, and other tissues by suturing it in place over the coil and opening, and allowing the slit that formed the flap to heal.
Other similar methods have involved the use of a plurality of leads which terminate at spaced active electrodes adapted to be inserted into the cochlea through the round window under the theory that selective activation of the electrodes may selectively stimulate different nerves in the cochlea and result in better sound discrimination by the person.
The use of any such method has presented the problem of properly positioning the electrodes with respect to the cochlea and holding the electrodes in that desired position. Experience has shown that positioning of the active electrode has often disrupted the delicate soft tissues of the cochlea (e.g., the basilar membrane or spiral ligament), and that a positioned electrode is sometimes inadvertently moved from the desired position during the operation. Also, it is sometimes necessary to replace the internal coil during a revision surgery which is done by again making an arcuate incision and folding back a flap of skin, muscle, and other tissues from over the internal coil, cutting the lead or leads connected to the internal coil, reconnecting a new internal coil, and suturing the flap in place over the new internal coil. Inadvertent movement of the electrodes has occurred during such severing and reconnecting of the leads which can result in damage to the cochlea or an improper new position for the electrodes. Such movement occurs because the anchor tissues packed around the leads do not attach firmly to the lead, and because of the slight force necessary to move the leads.